Material reducing machines for pulverizing, crushing and shredding various materials have been known for many years. In a particular type of such machine, a rotor is provided with loosely fitted ring hammers which tend to rotate with respect to the rotor itself upon impact with the material to be reduced. As a result of this movement, the wear on the hammer is distributed rather evenly over the cutting edges of the hammer as they sweep past the grate bars of the machine. The ring hammers tend to withdraw into the rotor upon encountering over-sized or extremely tough material, thereby avoiding some types of hammer failures which would occur if the hammers were mounted differently. U.S. Pat. Nos. 2,666,589 issued to Danyluke and 3,591,096 issued to DeFeo, both assigned to the assignee of the present application, disclose examples of prior art crushing machines embodying ring hammers. Another example is shown in U.S. Pat. No. 3,580,518 issued to Strom.
In prior art crushing machines of which the applicants are aware, the ring hammers have typically been cast from a type of manganese steel having good work-hardening characteristics. However, it has been found that cast ring hammers made from this material wear much more quickly than would be expected in some applications. More particularly, when the material being reduced is relatively soft, such as would be the case in coal granulators, the impact forces generated on the hammer surfaces apparently are insufficient to cause significant work hardening so that accelerated hammer wear is experienced. In addition, it has been observed over a number of years that cast hammers made from this and other materials are frequently rather difficult to balance as required prior to use so that excessive machine vibration is experienced. Finally, it has been observed that prior art ring hammers tend to wear in such a way that the hammer eventually achieves an almost smooth outer surface lacking any tooth-like protrusions which are desirable for effective crushing action. Without such tooth-like protrusions, the worn ring hammer has difficulty biting into the material and may actually fail to rotate with respect to the rotor, leading to uneven wear distribution around the circumference of the hammer.